Electronic amplifier control system



Dec. 16, 1947. cz.V L. FREDERICK Erm.

' VISILIJCTRONIG AMPLIFIR QONTROL SYSTEM Filed Jag. "2.9, 1945 ack' Curtis Ys y Patented Dec. 16, L1947A ELECTRONIC AMPLIFIER CONTROL SYSTEM i Carl L. Frederick, Williamsville, N. Y., and Roger C. Curtis, New Haven, Conn., assgnors to Dictaphone Corporation, New York, N. Y., a corporation of New York Application January 29, 1945, Serial No. 575,138

7 Claims.

This invention relates to electronic tube amplifier control systems for use in sound recording equipment and, more particularly, to arrangements in such amplifiers for automatically controlling the output volume in accordance with some function of the volume of the signal being amplified.

In sound recording systems to be used to make sound records from a wide variety of sources, such as telephone conversations, conferences, ofiice dictation, radio, and the like, in which the initial sounds have 'a wide range ,of average input sgnal'levels, e. g. from 120 db. to +10 db., and a wide range of 'volume variations at such average levels, e. g. 60 to 70 db., it isnecessary to amplify all signals, regardless of their diverse initial levels, to the` particular :average level necessary to operate the recorder element, e. g. 1G to 2 0 db., and to reduce the range .of volume variation in the amplified signals to a sufficiently narrow range so that the recorder element will not blast or .overcut, e. g. .a range of 425 to 30 db. In the past it has been customary to at tempt to achieve these results by the use of automatic volume control circuits.y commonly abbreviated A. V. C. circuits, These circuits have been of two general types: the forward-acting" type and the backward-acting type. The first is a circuit in which a Vportion of the input signal is amplified by .a separate amplifier, rectified, and fed in a negative sense as an operating grid bias to the control grid of one of the amplifier tubes of the variable-gain, e. g. remote-cut-off, type so as to control the gain of the amplifier system in accordance with the `volume of the input signal. In the backward-acting type, a portion of the output signal is rectified and fed back as an operating grid bias to the control grid of one of the amplifier tubes to control the amplication factor of the amplifier system in a similar manner. lThe first, or forward-acting type, system is relatively simple to design and operate, its operating characteristic tending to be degenerative, but it is subject to the disadvantage that the amplier output level is maintained practically constant regardless of input level so that, if its speedof response is sufciently fast to be of practical use, itcompletely robs the amplified signal of what might lbe called volume level perspective, i. e. a relative difference in volume level between loud and soft passages in the amplied signal, producing a substantially mono-level output, which usually is highly undesirable. The curve of such an operating characteristic is shown at B in Figure l. The second, `or "backward-acting type of automatic volume control is of the compression variety. It causes the amplifier to operate as a non-linear amplifier in which the range of volume change at its input is reduced in accordance with some predetermined ratio at its output. For example, assuming the amplier to have a 2 to 1 compression ratio, if the input signal varies through a range Vof 40 db., the output signal will only vary through a rangeof 2O db. The curve of ,Such an operating characteristic is shown at C in Figure l. Such a system has the advantage of maintaining volume level perspective while still reducing the volume range of the signal to be recorded to the volume range which can be handled by the recorder element. However, it has the disadvantage that its operating characteristic tends to be regenerative so that it is subject to motor-boating even when carefully designed unless it is provided with circuits having long time-constants, a design feature which renders it useless for many practical applications.

ln accordance with the present invention, compression of the amplified signal to a desired limited volume range throughout a wide range of input signal levels is accomplished with a forward-acting type of A. V. C. circuit by the provision of a novel control arrangement. A portion of the input signal potential is amplified, rectified to provide a D. C. potential and then is applied as an A. V. C. operating bias on one of the amplifier tubes, all in the usual manner. However, at the same time, a portion of this A. V. C. D. C. potential is fed back to control the amplification factor of the tube amplifying the portion of the input signal which is fed to the A. V. C. circuit. This novel arrangement permits effective A. V. C. operation throughout a very Wide range of input signal levels and also preserves volume level perspective in the amplified output signal.

Accordingly, it is an object of the present .invention to provide a novel forward-acting type A. V. C. system which incorporates the advantages but none of the disadvantages of the backward-acting type of A. V. C. system. It is a further object of the present invention to provide such an A. V. C. system in which the compression ratio may be varied throughout a wide range, and one which operates properly throughouta wide range of `input signal levels without blocking or overloading of the A. V. C. circuit components. These and other apparent objects and advantages of this invention are obtained by the means described in the following specifica- 3 tion and may be more readily understood by reference to the accompanying drawings wherein:

Figure 1 shows typical operating characteristic curves of various types of automatic volume control systems including those of an amplier system embodying the present invention; and

Figure 2 shows the schematic wiring diagram of an ampliiier system embodying the present invention.

Curve A represents the operating characteristic, i. e. the relationship of the input volume level to output volume level, of a theoretically perfect ampliiier not provided with any A. V. C. circuit, in which the output level varies directly with the input level. Curve B shows the operating characteristic curve of a typical amplifier equipped with the usual forward-acting type of A. V. C. circuit in which the output increases in direct proportion to the input level up to the point, point X in Figure 1, at which the A. V. C. circuit commences to function, after which the output is held at a substantially constant level regardless of further increase in the level of the input signal. Curve C is the operating characteristic curve of a typical ampliiier equipped with the usual type of "backward-acting A. V. C. circuit in which the output level increases directly with the input level up to the point X at which the A. V. C. circuit becomes effective and thereafter at a predetermined rate at some fixed ratio with respect to the rate of increase of the input signal, e. g. usually at a, ratio of 2 to 1. In accordance with the present invention, an operating characteristic curve of the type illustrated by curve C can be achieved with a forward-acting A. V. C. circuit and the slope of the curve above the point X, usually called the compression ratio of the A. V. C. circuit, can be adjusted to suit the particular needs of the recording being made.

Referring to Figure 2, the schematic wiring diagram of an electronic tube amplifier incorporating an A. V. C. circuit embodying the present invention is shown. This recording ampliiier includes a source of signal to be recorded, generally indicated by the enclosed area 00, such as a microphone, telephone, radio or the like; a preampliiier stage, generally indicated at |02, and including an input transformer T-I and a pentode amplifier tube V-I an A. V. C. controlled ampliiier stage, generally indicated at |04, including a variable-mu pentode tube V-2; an A. V. C. circuit comprising, an A. V. C. potential amplifying stage, generally indicated at |06, including a variable-mu pentode tube V-3, and an A. V. C. potential rectier circuit generally indicated at I 08, including a double diode rectier tube V-4; a, driver stage, generally indicated at I0, includn ing a triode amplifier tube V-5; a power ampli- Iier stage, generally indicated at l2, including a beam power pentode tube V-G; an output circuit, generally indicated at I I4, including an output transformer T-Z and a recorder element M| and a power supply circuit, generally indicated at I I6, including a full-wave rectifier tube V--l and a power transformer T--3, for supplying operating potentials to all of these amplifier portions.

The signal to be recorded from the signal source |00 is impressed through the input transformer T-I to the grid I2 of the pre-ampliiier pentode V-I of the pre-amplifier stage |02. This preamplifier pentode V--I is provided with the customary cathode bias resistor R-I with its bypass condenser C-I connected to a common ground '4 circuit I0, and its suppressor grid I5 is connected to its cathode II in the usual manner. Its plate I3 is connected through a plate load resistor R-Z and its screen grid I4 through a series resistor Rfto a suitable positive high voltage terminal I8 in the power supply H6, the screen grid I4 also being bypassed to ground by a bypass condenser C-2. The ampliiied signal appearing across the plate resistor Rf-Z is applied through a coupling condenser C--fi to the control grid 22 of the remote-cut-oi screen grid pentode tube V-2 in the amplifier A. V. C. stage |04 Where it is amplified in the normal manner so that the amplified signal appears across its plate load resistor R-4, which is connected to a positive high voltage terminal |22 in the power supply ||6. The suppressor grid 25 of the pentode V-Z is connected to the cathode 2| in the usual manner, the cathode 2| in turn being connected to the common ground circuit I0. The pentode V-Z is also provided with a screen grid series resistor R--5 and bypass condenser C5.

The amplilied signal from the pre-amplifier stage |02 is also fed to the A. V. C. potential amplifying stage |06 through a coupling condenser C-G to the control grid 32 of the pentode tube V-3 where it is amplied before being rectified by the A. V. C. rectier circuit |08. This A. V. C. potential amplifying tube V-3 is provided with the usual grid return resistor R-G, cathode bias resistor R--l with its bypass condenser C-`|, and screen grid resistor R-8 with its bypass condenser C-B and plate load resistor R-Q connected to a positive high voltage tap |20 in the power supply IIB. Its suppressor grid 35 is connected to its cathode 3| in the usual manner. The amplified A. V. C. potential which appears across the plate resistor R-Q of amplifier tube V-3 is applied through a coupling condenser C-9 to the A. V. C. rectifier tube V-4. This rectifier tube V-4 is a double diode connected in a voltage-doubler arrangement as shown in the diagram, so that the rectied A. V. C. pulsating D. C. potential appears across resistor R|0 shunted by a condenser C-|0. Briefly, this voltage doubler rectifier arrangement operates as follows: During the portion of the cycle of the signal imp-ressed on the grid 32 of A. V. C. potential amplifying tube V-3 when the grid is going negative, the current through this tube decreases, thus decreasing the voltage drop across plate resistor R-S and increasing the positive potential above ground of plate 33 and the side of the coupling condenser C-S connected thereto. This causes condenser C-9 to be charged through the cathode 43 and the anode 44 of one of the diodes of rectifier tube V-4 to a voltage equal to the peak value of this voltage change. During the next half cycle of the signal voltage, when the grid 32 of tube V-3 goes positive, the current through tube V-3 increases, thus increasing the voltage drop across the plate resistor R-9 and thereby decreasing the potential above ground of the plate 33 of tube V-3 and, thus, of the plate of condenser C-9 connected to this resistor. This, in turn, causes condenser C-S to discharge through cathode 4| and anode 42 of the other diode of rectiiier tube V-4 and condenser C-|0, thereby causing condenser C|0 to be charged to a Voltage substantially equal to the voltage change across the plate resistor R-9 of tube V-3 during one complete cycle of the signal voltage. This discharge of condenser C-9 prepares it for the next negative half cycle of the signal voltage and, at the same time, promitting control of the slope of the A. V. C. characteristic curve over wide limits, the novel A. V. C. circuit arrangement of the present invention makes possible an A. V. C. arrangement which operates over an exceptionlly Wide range of input signal levels, a feature which is of considerable importance in making satisfactory sound recordings from a wide Variety of signal sources. In the usual A. V. C. circuit, when the A. V. C. potential amplifying tube, i. e. tube V-3, is operated with sufliciently high operating potentials to quickly charge the A. V. C. condenser C-l and thus permit rapid operation of the A. V. C. circuit, an operating characteristic which is necessary in many sound recording systems, the A. V. C. potential amplifying tube V-3 tends to become saturated and block at high input signal levels if it is effective at low levels. On the other hand, if its circuit is adjusted so that it is operative on high input signal levels, it is ineffective when the input signal level is very low, thus limiting the input signal level range over which the A. V. C. circuit is effective. In accordance with the present invention, this disadvantage is overcome by the novel feature of feeding back a portion of the A. V. C. potential to the A. V. C. potential amplifying tube so that its range of effective operation is considerably extended.

As many embodiments may be made in the above invention and as many changes may be made in the embodiment above described, it is to be understood that all matters hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative only and not in a limiting sense.

We claim:

1. In an electronic tube amplifier system for sound recording, an A. V. C. compression circuit, comprising, in combination, first amplifier means including A. V. C. means responsive to a D. C. bias potential for controlling its gain, means for applying the signal to be amplified to the input circuit of said first amplifier means, means for connecting the output circuit of said first amplifier means to the output circuit of said amplifier system, second amplifier means including A. V. C. means responsive to a D. C. bias potential for controlling its gain, means for applying the input signal to the input circuit of said second amplifier means, means for converting the amplified signal output of said second amplifier means into a D. C. potential, means for applying said D. C. potential as an A. V. C. potential to control the amplification of said first amplifier means, and means for applying a predetermined portion of said D. C. potential as an A. V. C. potential to control the amplification of said second amplifier means.

2. In an electronic tube amplifier system for sound recording, an A. V. C. compression circuit, comprising, in combination, first amplifier means including A. V. C. means responsive to a D. C. bias potential for controlling its gain, means for applying the signal to be amplified to the input circuit of said first amplifier means, means for connecting the output circuit of said first amplifier means to the output circuit of said amplifier system, second amplifier means including A. V. C. means responsive to a D. C. bias potential for controlling its gain, means for applying the input signal to the input circuit of said second amplifier means, means for converting the amplifier signal output of said second amplifier means into a D. C. potential, means for applying said D. C. potential as an A. V. C. potential to control the amplification of said first amplifier means, means for applying a predetermined portion of said D. C. potential as an A. V. C. potential to control the amplification of said second amplifier means, and means for biasing said converter means to render it inoperative until the amplified signal output from said second amplifier means reaches a predetermined value.

3. An A. V. C. compression circuit for use in an electronic tube amplilier system, comprising, in combination, a first A. V. C. amplifier circuit including a first variable-mu pentode tube, a second A. V. C. amplifier circuit including a second variable-mu pentode tube, an input circuit for applying the signal to be amplified to the input of said first A. V. C. amplifier circuit and to the input of said second A. V. C. amplifier circuit, an output circuit for connecting the output circuit of said first amplifier circuit to the remainder of said amplifier system, rectifier means for changing the amplified signal output of said second amplifier circuit into a D. C. A. V. C. potential, means connecting the D. C. output of said rectifier means to the control grid circuit of said first pentode tube as an A. V. C. grid biasing potential, and Voltage control means applying a predetermined portion of the D. C. output of said rectifier means to the control grid of said second pentode as an A. V. C. grid biasing potential to permit adjustment of the compression ratio of said A. V. C. circuit.

4. An A. V. C. compression circuit for use in an electronic tube amplifier system, comprising, in combination, a rst A. V. C. amplifier circuit including a first variable-mu pentode tube, a second A. V. C. amplifier circuit including a second Variable-mu pentode tube, an input circuit for applying the signal to be amplified to the input of said first A. V. C. amplifier circuit and to the input of said second A. V. C. amplifier circuit. an output circuit for connecting the output circuit of said rst amplifier circuit to the remainder of said amplifier system, rectifier means for changing the amplified signal output of said second amplifier circuit into a D. V. A. V. C. potential, means connecting the D. C. output of said rectifier means to the control grid circuit of said first pentode tube as an A. V. C. grid biasing potential, Voltage control means applying a predetermined portion of the D. C. output of said rectifier means to the control grid of said second pentode as an A. V. C. grid biasing potential to permit adjustment of the compression ratio of said A. V. C. circuit, and means for applying a positive D. C. bias to the cathode of said rectifier means to permit the amplified signal output of said amplifier circuits to reach a predetermined level before said A. V. C. circuit becomes effective.

5. In an electronic tube amplifier system, an automatic volume compression circuit, comprising, in combination, first amplifier means including means for controlling its gain in accordance with a bias potential, means applying the signal to be amplified to the input circuit of said first amplifier means, means connecting the output circuit of said Iirst amplifier means to the output circuit of said amplifier system, second amplifier means including means for controlling its gain responsive to a bias potential, means applying the input signal to the input circuit of said second amplifier means, means applying a lirst predetermined portion of the amplified signal output of said second amplifier means to the gain control means of said first amplifier means, and means applying a second predetermined portion of the amplified signal output of said second amplier means to the gain control means of said second amplifier means.

6. In an electronic tube amplier system for sound recording, the method of automatically compressing the magnitude of the output signal in accordance with the magnitude of the input signal which comprises the steps of performing a first ampliiication operation on said input signal, performing a second and separate amplification operation on said input signal, producing a control potential in accordance with the magnitude of the amplied output signal of said second amplication operation, controlling the gain of said rst amplification operation with said control potential, simultaneously controlling the gain of said second amplication operation With said control potential, and applying the amplified signal output of said first amplification operation to the output portion of said amplier system.

'7. In an electronic tube amplier system for sound recording, the method of automatically compressing the magnitude of the output signal in accordance with the magnitude of the input signal which comprises the steps of performing 10 a iirst amplication operation on said input signal, performing a second and separate amplification operation on said input signal, rectifying the output signal of said second amplication operation to produce a D. C. control potential, applying said D. C. control potential to control the gain of said rst amplication operation, simultaneously applying a portion of said D. C. control potential to control the gain of said second amplication operation, and applying the amplified signal output of said first amplification operation to the output portion of said amplier system.

CARL L. FREDERICK.

ROGER C. CURTIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,303,358 Hoover, Jr Dec. 1, 1942 1,959,062 Place May 15, 1934 2,096,759 Purington Oct. 26, 1937 2,208,665 Crabtree July 23, 1940 

